X-shape pulses hold together

By
Eric Smalley,
Technology Research NewsThere's more to some light pulses than
meets the eye.

The lengths of the lightwaves that make up a light pulse determine
its color, and the way the waves spread out determines its shape. Lightwaves
of slightly different lengths, or colors, travel at slightly different
speeds unless they are in a vacuum, giving light pulses a tendency to
spread as they progress through materials like glass and biological tissue
or through gases like air.

A team of researchers in Italy and Lithuanian has found that under
certain conditions a pulse of light can form an X shape that does not
spread out.

These intense pulses could be used to improve microscopes and
techniques for etching microscopic features into materials. The method
could be used in any task "where it is necessary to have an intense and
localized hot source of power [that] keeps its shape... inside the material
where the light propagates," said Paolo Di Trapani, a professor of optics
at the University of Insubria in Italy.

The researchers found that the intense pulses formed spontaneously
when they fired conventional laser pulses through a lithium triborate
crystal. The crystal structure causes a conventional pulse to reshape
into an X-shaped wave. The pulse has an intense central hump that is balanced
by a pair of crossed cones.

It is the first evidence of stable three-dimensional light bullets,
or light pulses that hold their shape, according to Di Trapani.

Researchers have been trying to make light bullets for decades,
inspired in part by occasional observations of similar effects in nature.
Under certain conditions, hump-shaped water waves in shallow water can
persist longer than ordinary waves. These persistent waves, or solitons,
result from interactions between the dynamics of the water and the surface
beneath the water.

Waves ordinarily spread out over time and eventually dissipate,
but in shallow water the lower portion of a wave is slowed by friction
from the bottom while the top of the wave continues at its original speed,
causing the wave to form a crest and eventually break. Occasionally these
forces balance each other, allowing a wave to preserve its shape.

Similarly, a light pulse traveling through a material experiences
contrary forces that can be balanced. A pulse is dispersed because the
different wavelengths of its components are bent, or refracted, to different
degrees by the material the pulse is traveling through, causing them to
move at different speeds. Light's intensity also affects diffraction,
with the stronger light at a pulse's center bending more sharply, causing
the pulse to squeeze inward.

By carefully tuning a light pulse and the material it travels
through, researchers can produce light-pulse solitons. However, these
solitons don't remain stable for very long. "Recent results have indeed
shown that the instability grows pretty fast," said Di Trapani.

The key to the researchers' X-shaped light bullets is that the
frequency of each component of a pulse also depends on its angle, and
the angles formed by the X shape lead to a particular dispersion of the
pulse's components. This angular dispersion pattern is exactly the opposite
of the dispersion produced by the crystal, so the two types of dispersion
cancel each other, preserving the pulse's size and shape.

The researchers produced X waves by firing infrared laser pulses
lasting from 100 to 200 femtoseconds into a 22-millimeter-long section
of lithium triborate crystal. When the pulses entered the crystal, they
were compressed to about 20 femtoseconds and reshaped into the X wave.
A femtosecond is one million billionth, or quadrillionth, of a second.

The researchers' next steps are demonstrating that the X waves
are suitable for practical applications, and extending the study of the
phenomenon to quantum optics, said Di Trapani. The phenomenon should also
apply to acoustics, according to Di Trapani.

Using the X waves practically will require finding appropriate
methods of generating the pulses, developing lenses, mirrors and holograms
capable of controlling the pulses, and developing techniques for tracking
and measuring the pulses, said Di Trapani. The X-shaped light bullets
could be used in practical applications in two to five years, he said.

Di Trapani's research colleagues were G. Valiulis and A. Piskarskas
of Vilnius University in Lithuania, O. Jedrkiewicz and J. Trull of the
University of Insubria in Italy, C. Conti from the Italian National Institute
of Material Physics, and S. Trillo from the Italian National Institute
of Material Physics and the University of Ferrara in Italy.

The work appeared in the August 29, 2003 issue of Physical Review
Letters. The research was funded by the Italian Ministry for Education,
University and Research; the United Nations Educational, Scientific and
Cultural Organization; the Lithuanian Science and Studies Foundation;
the Spanish Ministry of Education, Culture and Sports; and the Tronchetti
Provera Foundation in Italy. Technical assistance was provided by Light
Conversion Ltd. of Lithuania.